Water-Soluble Or Water-Swellable Polymers As Water Loss Reducers In Cement Slurries

ABSTRACT

This invention relates to water-soluble or water-swellable polymers, containing a) 25-35 mol. % of one or more recurrent structural units of formula (1), where R 1  and R 2  represent hydrogen, methyl or ethyl, A represents a linear or branched C 1 -C 12 -alkylene, and Q +  stands for H + , NH 4   + , Li + , Na + , K + , ½Ca ++ , ½Mg ++ , ½Zn ++ , ⅓Al +++ , or organic ammonium ions of the formula [HNR 5 R 6 R 7 ] + , b) 3 to 8 mol. % of one or more recurrent structural units of formula (2), where R 1  represents hydrogen, methyl, or ethyl, X +  stands for H + , NH 4   + , Li + , Na + , K + , ½Ca ++ , ½Mg ++ , ½Zn ++ , ⅓Al +++ , or organic ammonium ions of the formula [HNR 5 R 6 R 7 ] + , B is a linear or branched alkylene group with 1 to 6 carbon atoms, and n is a whole number between 0 and 5, and c) 57 to 72 mol. % of a (meth)acrylamide.

The present invention relates to water-soluble or water-swellablepolymers based on acryl-, methacryl- or ethacrylamidoalkylsulfonic acidor salts thereof and carboxyalkyl acrylate, methacrylate or ethacrylateor oligomers of these carboxyl compounds, and acrylamides oralkylacrylamides, to a process for preparing these polymers and to theuse thereof as water loss reducers in cement slurries for cementing deepwells for reducing the water loss at the wellbore wall (fluid lossadditives).

In deep wells for developing mineral oil and natural gas deposits, theuse of cement slurries has long been known. Once the wellbore hasreached a certain depth, what are called feed tubes are introduced intothe wellbore. For this purpose, the feed tubes have to be fixed, meaningthat a cement slurry is pumped into the cavity between the rock and thefeed tubes and hardens to give a solid rock. The cement rock which formshas to be impermeable to gases and liquids, in order that no gas and/oroil can flow out of the reservoir rock into other sections or up to thesurface. High demands are made on the cement slurry to be pumped. Itshould have good pumpability, i.e. minimum viscosity, and neverthelessnot exhibit any separation. The release of water from the cement slurryto the porous rock during the pumping operation should be low, in orderthat no thick filtercakes form at the wellbore wall, which wouldincrease the pump pressure because of the annular space constriction tosuch a high degree that the porous rock breaks up. Moreover, the cementslurry, in the case of excessive water release, would not set optimallyand would become permeable to gas and oil. On the other hand, the cementshell which forms in the annular space must attain sufficient strengthvery quickly, and no shrinkage, which leads to flow channels for gas,oil and water, may occur in the course of setting. Optimal adjustment ofthe properties of the cement slurry is possible only by means ofadditives. The most important additives are retardants, accelerators,dispersants and water loss reducers.

Effective water loss reducers used in practice for cement and gypsumslurries are a wide variety of different polymers, copolymers andcombinations thereof. The first effective products, which are stillbeing used even now, were cellulose ethers based on hydroxyethylcellulose and carboxymethyl hydroxyethyl cellulose. Owing to thermalinstability, these lose efficiency at wellbore temperatures above 100°C. (212° F.). If the temperature rises to about 120° C. to 150° C.,thermal breakdown of these biogenic substances commences. As a result,many different fully synthetic thermally stabilized polymers have beendeveloped and are still being used nowadays at the differenttemperatures and salinities of the cement slurry.

Polymers as additives for reducing the water loss from cement slurriesare well known in the literature, although many have very limitedactivity in the temperature range between 30° C. (86° F.) and 200° C.(392° F.).

U.S. Pat. No. 2,614,998 describes the use of partly hydrolyzedpolyacrylamide (polyacrylamide-co-acrylic acid) as water loss-reducingpolymer. However, these polymers can lead to significant delays in thesetting time of the cement and show only low efficacy at hightemperatures.

U.S. Pat. No. 2,865,876, U.S. Pat. No. 2,905,565 and U.S. Pat. No.3,052,628 describe the use of sulfonated polymers as additives. Thepolymers and copolymers described therein differ distinctly in terms ofcomposition from the copolymers of the invention and have gained notechnical significance at all.

U.S. Pat. No. 5,472,051 describes copolymers of acryloyldimethyltaurateand acrylic acid having molecular weights of less than 5000 g/mol.However, these polymers can lead to severe delays in the setting time ofthe cement and, because of the molecular weight, show high water lossesat high temperatures.

WO-99026991 and EP-1045869 teach copolymers of acryloyldimethyltaurateand acrylamide, but these polymers, in direct comparison with thecopolymers of the invention, exhibit poorer performance properties belowa temperature of 50° C. (122° F.) (comparative example 1).

U.S. Pat. No. 4,015,991 describes a polymer prepared by polymerizationof acryloyldimethyltaurate and acrylamide in water, wherein at least 20%of the acrylamide units have to be hydrolyzed subsequently to acrylicacid or a salt of acrylic acid. The copolymer described in the examplesof U.S. Pat. No. 4,015,991 has formed through the polymerization of116.4 g (0.56 mol) of acryloyldimethyltaurate and 14.7 g (0.207 mol) ofacrylamide in water. After at least 20% of the acrylamide units havebeen hydrolyzed to acrylic acid, U.S. Pat. No. 4,015,991 claims thefollowing copolymer:

wherex is 73.8 mol %y is max. 21.8 mol %z is at least 4.4 mol %, depending on y.

However, the disadvantage of this polymer is an unwanted influence onthe cement properties (reduction in hardened cement strength) and thedelaying effect on the solidification of the cement. A further problemis the restricted temperature range for use as a water loss-reducingpolymer. At 176.7° C. (350° F.), it is demonstrably inactive(comparative examples 5 and 6). U.S. Pat. No. 4,015,991 showed that itis not possible by aqueous polymerization of acryloyldimethyltaurate andacrylamide without a hydrolysis step to produce any copolymer suitablefor application purposes.

EP-0116671 discloses the introduction of 5%-60% by weight of vinylamides(e.g. N-vinylmethylacetamide) into acryloyldimethyltaurate-containingpolymers. In this way, the high-temperature range of the application wassignificantly extended. However, these polymers have poorer performanceproperties at temperatures below a temperature of 50° C. (122° F.).

U.S. Pat. No. 5,025,040 describes copolymers of acryloyldimethyltaurate,acrylamide and at least 20% N-vinylimidazole.

U.S. Pat. No. 4,931,489 discloses copolymers of substituted acrylamidesand N-vinylimidazoles, without the use of acryloyldimethyltaurate.

EP-0217608, U.S. Pat. No. 4,555,269 and EP-0157055 describe a copolymerof acryloyldimethyltaurate and dimethylacrylamide in a molar ratio of1:4 to 4:1 as a fluid loss additive for salt-containing (about 10% byweight) cement slurries and the use of acryloyldimethyltaurate andacrylic acid in a molar ratio of 1:4 to 4:1 for the same purpose.

Polymers based on acryloyldimethyltaurate or salts thereof are alreadyknown. No solution satisfactory for application purposes for atemperature range between 30° C. (86° F.) and 200° C. (392° F.) on thebasis of the monomers disclosed in U.S. Pat. No. 4,015,991 has beendescribed to date.

Polymers based on acryloyldimethyltaurate, acrylic acid and acrylamideare likewise known:

EP-0244981 discloses polymers based on acryloyldimethyltaurate, acrylicacid and acrylamide as a soil-repellent sealing formulation. Thesepolymers are prepared by the free-radical polymerization of the monomersin an aqueous medium. Polymers prepared according to examples 1-8 inEP-0244981, because of their polymer architecture (comparative examples2, 3 and 4), demonstrably do not bring about any reduction in water losswhen they are used as additive in cement slurries.

Polymers based on sulfonates and acrylamide, according to U.S. Pat. No.4,800,071, are employed as filtration aids in order to remove sparinglysoluble calcium salts from aqueous phosphoric acid solutions. No use ofthese polymers as water loss reducers in cement slurries for cementingdeep wells has been disclosed.

U.S. Pat. No. 4,342,653 discloses polymers based onacryloyldimethyltaurate and acrylamide. These polymers are used asprecipitation aids for aqueous dispersions. The precipitation aidsdescribed should have between 1 and 35 mol % of repeat units ofacryloyldimethyltaurate, and a Brookfield viscosity of at least 2·10⁻³Pa·s. The example adduced in U.S. Pat. No. 4,342,653 features anacrylamide content of 94 mol %. Such a high molar acrylamide contentdoes not lead to any reduction in water loss in cement slurries.

JP-11310751 describes polymers based on 10-90 mol % ofacryloyldimethyltaurate, 0-90 mol % of acrylamide and 0-30 mol % offurther copolymerizable monomers suitable for paper coatings, adhesivesand emulsion-based adhesives. These polymers are prepared by thefree-radical polymerization of the monomers in an aqueous medium.Polymers prepared by the process in JP-11310751, because of theirpolymer architecture, do not bring about any reduction in water losswhen they are used as additive in cement slurries.

JP-63060240 discloses polymers based on acryloyldimethyltaurate,acrylamides and sodium acrylate, which are used as precipitants in goldsuspensions or gels for gold separation. The polymer specified in theexample contains 13.3 mol % of AMPS, 13.3 mol % of sodium acrylate and73.4 mol % of acrylamide, and was prepared with the aid of an aqueousfree-radical polymerization. However, the disadvantage of these polymersis an unwanted influence on the cement properties (reduction in hardenedcement strength) and the delaying effect on the solidification of thecement. As an additive in cement slurries, the polymer also does notexhibit any effects which contribute to reduction in water loss incement slurries.

Additives on the market which are used as water loss reducers in cementslurries for cementing deep wells are acryloyldimethyltaurate andcopolymers thereof (e.g. HOSTAMER® 4707 from Clariant). However, thesein turn have the disadvantage that they lead to an increase in waterloss below a temperature of 180° C. (356° F.).

The multitude of polymers developed for reduction of water release makesit clear that it is always problematic to formulate a cement slurrywhich is optimal for application purposes for a temperature rangebetween 30° C. (86° F.) and 200° C. (392° F.). A significant influenceon the suitability thereof is exerted by the temperature of the wellboresection which is being prepared for cementing. The polymers optimizedfor different temperatures constitute major logistical problems becausea certain stock of extra water loss-reducing polymers always has to bekept at sites all over the world.

It was therefore an object of the present invention to providesubstances which can help to achieve improved control of liquid loss inthe cement slurries for cementing wellbores at temperatures between 80°F. and 300° F.

It has now been found that, surprisingly, the performance propertiesrequired can be achieved through the copolymerization of acryl-,methacryl- or ethacrylamidoalkylsulfonic acid or salts thereof and withcarboxyalkyl acrylate, methacrylate or ethacrylate or oligomers of thesecarboxyl compounds, and with acrylamides or alkylacrylamides, when thepolymerization process is altered and the subsequent hydrolysis isdispensed with, and hence a novel polymer which has not been describedto date is prepared. This polymer also does not have any set-delayingproperties below 50° C.

The present invention provides water-soluble or water-swellable polymershaving a k value of 100 to 300, measured in 0.5% by weight solution indistilled water, containing

-   a) 25-35 mol % of one or more recurrent structural units of the    formula (1)

-   -   in which    -   R¹, R² are hydrogen, methyl or ethyl,    -   A is linear or branched C₁-C₁₂-alkylene, and    -   Q⁺ is H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺, ½Ca⁺⁺, ½Mg⁺⁺, ½Zn⁺⁺, ⅓Al⁺⁺⁺,        organic ammonium ions of the formula [HNR⁵R⁶R⁷]⁺ where R⁵, R⁶        and R⁷ may each independently be hydrogen, a linear or branched        alkyl group having 1 to 22 carbon atoms, a linear or branched,        mono- or polyunsaturated alkenyl group having 2 to 22 carbon        atoms, a C₆-C₂₂-alkylamidopropyl group, a linear        monohydroxyalkyl group having 2 to 10 carbon atoms or a linear        or branched dihydroxyalkyl group having 3 to 10 carbon atoms,        and where at least one of the R⁵, R⁶ and R⁷ radicals is not        hydrogen, or mixtures of these ions,

-   b) 3 to 8 mol % of one or more recurrent structural units of the    formula (2)

-   -   in which    -   R¹ is hydrogen, methyl or ethyl,    -   X⁺ is H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺, ½ Ca⁺⁺, ½ Mg⁺⁺, ½ Zn⁺⁺, ⅓ Al⁺⁺⁺,        organic ammonium ions of the formula [HNR⁵R⁶R⁷]⁺ where R⁵, R⁶        and R⁷ may each independently be hydrogen, a linear or branched        alkyl group having 1 to 22 carbon atoms, a linear or branched,        mono- or polyunsaturated alkenyl group having 2 to 22 carbon        atoms, a C₆-C₂₂-alkylamidopropyl group, a linear        monohydroxyalkyl group having 2 to 10 carbon atoms or a linear        or branched dihydroxyalkyl group having 3 to 10 carbon atoms,        and where at least one of the R⁵, R⁶ and R⁷ radicals is not        hydrogen, or mixtures of these ions,    -   B is a chemical bond, or a linear or branched alkylene group        having 1 to 6 carbon atoms, and    -   n is an integer from 0 to 5,        and

-   c) 57 to 72 mol % of one or more recurrent structural units of the    formula (3)

-   -   in which    -   R¹ is hydrogen, methyl or ethyl, and    -   R³ and R⁴ are each independently hydrogen, methyl, ethyl,        n-propyl, isopropyl or butyl.

The invention further provides for the use of the polymers of theinvention as water loss reducers for cement formulations. Preferably,0.05% to 5% by weight of the polymers, based on the weight of the cementformulation, is used in cement formulations.

In a polymer of the invention, it is possible in each case for differentstructural units of the formula (1) and/or of the formula (2) and/or ofthe formula (3) to be present. A polymer of the invention may, forexample, contain several structural units of the formula (1) whichdiffer from one another by different counterions Q⁺. A polymer of theinvention may, for example, also contain several structural units of theformula (2) which differ from one another by different counterions X⁺. Apolymer of the invention may, for example, also contain severalstructural units of the formula (3) which differ by different R¹, R³ andR⁴ radicals. The R¹ radicals in the structural units of the formulae(1), (2) and (3) may be identical or different in all the structuralunits.

The weight-average molecular weights of these polymers are preferably300 000 to 5 000 000, more preferably 500 000 to 4 000 000, especially600 000 to 2 500 000 g/mol. The weight-average molecular weights can bedetermined with the aid of gel permeation chromatography (GPC). Theprocedure for determining the weight-average molecular weight with theaid of GPC is described in detail in “Makromolekulare Chemie: EineEinführung” [Macromolecular Chemistry: An Introduction] by Bernd Tieke,Wiley-VCH, 2nd fully revised and extended edition (Sep. 9, 2005)ISBN-10: 3527313796 in chapter 3. The polymers were analyzed against apolystyrenesulfonate standard.

Indicators used for the molecular weight are the relative viscosity andthe k value. To determine the k value, the copolymer is dissolved indistilled water in a concentration of 0.5% by weight and the outflowtime at 20° C. is determined by means of an Ubbelohde viscometer. Thisvalue gives the absolute viscosity of the solution (η_(c)). The absoluteviscosity of the solvent is (η₀). The ratio of the two absoluteviscosities gives the relative viscosity:

$Z = \frac{n_{c}}{n_{0}}$

The relative viscosity Z and the concentration C can be used todetermine the k value by means of the following equation:

${{Lg}\mspace{14mu} z} = {\left( {\frac{75*k^{2}}{1 + {1.5{kc}}} + k} \right)*c}$

The k value of the polymers of the invention is from 100 to 300,preferably from 150 to 270 and especially preferably from 180 to 250.

In the structural units of the formula (1), R¹ is preferably hydrogen ormethyl and more preferably hydrogen.

In the structural units of the formula (1), A is preferably a structuralunit of the formula —C(CH₃)₂—CH₂—.

The structural units of the formula (1) are preferably derived frommonomers from the group consisting of acryloyldimethyltaurate,acryloyl-1,1-dimethyl-2-methyltaurate, acryloyltaurate,acryloyl-N-methyltaurate, preferably acryloyldimethyltaurate.

Preferably, the neutralization level of the structural units of theformula (1) is from 50.0 to 100 mol %, more preferably from 80.0 to 100mol %, especially preferably from 90.0 to 100 mol % and exceptionallypreferably from 95.0 to 100 mol %.

In the structural units of the formula (1), Q is partly H+ ifneutralization is incomplete. In the case of partial or completeneutralization, Q+ is preferably selected from NH₄ ⁺, Ca²⁺ and Na⁺ andmixtures of these ions. More preferably, the counterion Q other than H⁺is NH₄ ⁺.

In the structural units of the formula (2), R¹ is preferably hydrogen ormethyl and more preferably hydrogen.

In the structural units of the formulae (2), B is preferably a chemicalbond or a structural unit of the formula —CH₂—CH₂—.

In the structural units of the formula (2), n is preferably 0 to 5 andmore preferably 0 to 3 and especially preferably 0 or 1.

Preferably, the proportion of structural units of the formula (2) inwhich n is 0 within component b) of the polymers of the invention is atleast 70.0 mol %, more preferably at least 80.0 mol %, especiallypreferably at least 90.0 mol %, exceptionally preferably at least 95.0mol %.

More preferably, in the structural units of the formula (2), B is achemical bond or the —CH₂CH₂— group.

In a further preferred embodiment, in the structural units of theformula (2), B is a chemical bond or the —CH₂CH₂— group and n is aninteger from 1 to 5, and preferably from 1 to 3 and more preferably 1.

In the structural units of the formula (2), X is partly H+ ifneutralization is incomplete. In the case of partial or completeneutralization, X is preferably selected from NH₄ ⁺, Ca²⁺ and Na⁺ andmixtures of these ions. More preferably, the counterion X that is not H⁺is NH₄ ⁺

In a further particularly preferred embodiment of the invention, X⁺ isH⁺.

In a further particularly preferred embodiment of the invention, thepolymers of the invention contain several different structural units ofthe formula (2), where the counterions X⁺ in some structural units ofthe formula (2) are defined as H⁺ and the counterions X⁺ in the otherstructural units of the formula (2) are defined differently than H⁺, andpreferably as NH₄ ⁺.

In a further particularly preferred embodiment of the invention, thepolymers of the invention contain several different structural units ofthe formula (2) which differ in terms of R¹, b and/or n.

These structural units are preferably derived from monomers from thegroup consisting of methacrylic acid, acrylic acid, carboxyethylacrylate and higher oligomers of the formula (2) in which n is aninteger of 2 or more.

Especially preferably, the structural units are derived from methacrylicacid, acrylic acid and carboxyethyl acrylate. Particular preference isgiven to methacrylic acid and acrylic acid.

The structural units of the formula (3) are preferably derived frommonomers from the group consisting of acrylamide, N-methylacrylamide,N-ethylacrylamide, N,N-diethylmethacrylamide, N,N-diethylacrylamide,N,N-dimethylmethacrylamide, N,N-dimethylacrylamide,N-isopropylacrylamide, N-tert-butylacrylamide and N-butylacrylamide,preferably acrylamide, methacrylamide, N,N-diethylacrylamide,N,N-dimethylacrylamide, isopropylacrylamide, more preferably acrylamide,methacrylamide, N,N-dimethylacrylamide, especially preferablyacrylamide.

In a further embodiment of the invention, the polymers of the inventionpreferably contain

-   a) 27.5 to 32.5 mol % of structural units of the formula (1),-   b) 4.5 to 7.5 mol % of structural units of the formula (2), and-   c) 60 to 68 mol % of structural units of the formula (3).

In a further preferred embodiment of the invention, the structural unitsof the formula (1) are derived from acryloyldimethyltaurate, those ofthe formula (2) are derived from acrylic acid and those of the formula(3) are derived from acrylamide.

Particularly preferred polymers of the invention contain structuralunits of

-   a) 25 to 35 mol % of acryloyldimethyltaurate, and-   b) 3 to 8 mol % of acrylic acid, and-   c) 57 to 72 mol % of acrylamide.

A further embodiment of particularly preferred polymers of the inventioncontains structural units of

-   a) 27.5 to 32.5 mol % of acryloyldimethyltaurate, and-   b) 4.5 to 7.5 mol % of acrylic acid, and-   c) 60 to 68 mol % of acrylamide.

In a further preferred embodiment of the invention, the polymers of theinvention do not contain any cationic structural units.

The distribution of the different structural units in the polymers ofthe invention may be random, blockwise or in alternating or gradientform. The polymers of the invention are generally prepared byfree-radical polymerization.

Free-radical polymerizations are common knowledge to those skilled inthe art and are described in detail in standard literature works, forexample in “Makromolekulare Chemie: Eine Einführung” by Bernd Tieke,Wiley-VCH, 2nd fully revised and extended edition (Sep. 9, 2005)ISBN-10: 3527313796.

The polymers of the invention are preferably prepared by means offree-radical precipitation polymerization in a polar solvent or solventmixture. This involves dissolving or dispersing the correspondingmonomers, for example, in the polar solvent or solvent mixture andinitiating the polymerization in a manner known per se, for example byadding a free radical-forming compound. It is possible here, forexample, for the initially charged monomers to be polymerized“directly”. Alternatively, they may also be neutralized prior to thepolymerization, for example by reacting acidic groups in monomers usedwith bases prior to the polymerization, forming the counterions Q⁺ ofthe structural units of formula (1) or X⁺ of the structural units offormula (2). Instead of the neutralization of the monomers prior to thepolymerization, however, it is also possible to neutralize the polymerswith the bases on completion of polymerization.

The present invention therefore further provides a process for preparingthe polymers of the invention, wherein monomers from which thestructural units of components a) to c) derive are free-radicallypolymerized in a polar solvent or solvent mixtures comprising suchsolvents, and the monomers are optionally neutralized prior to thepolymerization, or the polymer is neutralized after the polymerization,with ammonia or organic amines or an Li⁺-, Na⁺-, K⁺-, Ca⁺⁺-, Mg⁺⁺-,Zn⁺⁺- or Al⁺⁺⁺-containing base, preferably with the correspondinghydroxides or carbonates and more preferably with hydroxides.

Useful polymerization initiators include all free radical-formingsubstances; as well as typical diazo compounds and peroxy compounds,initiation is also possible by means of redox initiators, aphotoinitiator or by means of high-energy radiation (UV, neutrons,plasma). In contrast to aqueous free-radical polymerization, there isonly a minor dependence of the product on the type and amount of theinitiator system used.

In a preferred embodiment of the process for preparing the polymers ofthe invention, the free-radical precipitation polymerization is effectedin a polar solvent or solvent mixture which is characterized in that thesolvent or solvent mixture has a boiling point of 60 to 110° C.,preferably of 60 to 85° C., more preferably of 70 to 85° C.

In a further preferred embodiment of the process for preparing thepolymers of the invention, the polar solvent contains:

-   d) water    and-   e) one or more further polar solvents, preferably alcohols, dialkyl    ketone and cyclic ethers, more preferably alcohols, dialkyl ketone    and especially preferably alcohols.

In a further preferred embodiment of the process for preparing thepolymers of the invention, component e) contains one or more polarsolvents selected from the group of methanol, ethanol, 1-propanol,2-propanol, 2-methyl-2-propanol, 2-butanol, dimethyl ketone, diethylketone, tetrahydropyran, tetrahydrofuran, 2-methyltetrahydrofuran,1,3-dioxane, 1,4-dioxane, preferably ethanol, 1-propanol, 2-propanol,2-methylpropan-2-ol, 2-butanol, dimethyl ketone, tetrahydrofuran,2-methyltetrahydrofuran, 1,3-dioxane, more preferably 2-propanol,2-methylpropan-2-ol, dimethyl ketone, tetrahydrofuran,2-methyltetrahydrofuran, 1,3-dioxane, especially preferably2-methylpropan-2-ol, dimethyl ketone and exceptionally preferably2-methylpropan-2-ol.

In the process for preparing the polymers of the invention, it ispossible for various polar solvents of component e) to be present. Onepolar solvent of the invention in component e) may, for example,comprise 2-methylpropan-2-ol. A further solvent of the invention incomponent e) may, for example, comprise a mixture of 2-methylpropan-2-oland dimethyl ketone. A further solvent of the invention in component e)may, for example, comprise a mixture of 2-methylpropan-2-ol andtetrahydrofuran.

In a particular embodiment of the process for preparing a polymer of theinvention, the polar solvent mixture contains 0.5% to 20% by weight,preferably 0.5% to 10% by weight and more preferably 1% to 8% by weightof water and exceptionally preferably 2% to 5% by weight of water.

In a further particular embodiment of the process for preparing apolymer of the invention, the polar solvent mixture contains 5% to 99.5%by weight, preferably 10% to 99.5% by weight and more preferably 30% to99.5% by weight of 2-methylpropan-2-ol.

In a further particular embodiment of the process for preparing apolymer of the invention, the polar solvent mixture contains 0.5% to 20%by weight of water, 7.5% to 92% by weight of 2-methylpropan-2-ol and7.5% to 92% by weight of dimethyl ketone, preferably 0.5% to 7.5% byweight of water, 20% to 89.5% by weight of 2-methylpropan-2-ol and 10%to 79.5% by weight of dimethyl ketone.

In an exceptionally preferred process for preparing an inventivepolymer, 27.5 to 32.5 mol % of acryloyldimethyltaurate, 4.5 to 7.5 mol %of acrylic acid and 60 to 68 mol % of acrylamide are free-radicallypolymerized in a polar solvent mixture, preferably a mixture of 1% to 8%by weight of water and 92% to 99% by weight of 2-methylpropan-2-ol, andthe monomers are optionally neutralized prior to the polymerization, orthe polymer is neutralized after the polymerization, with ammonia,ammonium carbonate, sodium hydroxide, sodium carbonate, preferably withammonia.

The polymers are obtained as a white voluminous precipitate in the polarsolvent mixture. For isolation, it is possible to use all the customaryevaporation and drying isolation processes. More particularly, the polarsolvent mixture can be removed from the product by a pressure filtrationor distillation. A small residue of the polar solvent mixture is of noconcern, either for safety reasons or for application reasons.

The invention further provides a method for cementing deep wells using acement slurry containing the polymer of the invention in a concentrationof 0.01% to 5% bwoc (by weight of cement), preferably 0.05% to 0.9%bwoc. Further components of the cement slurry are waters of differentsalinity and cement. In addition, it is possible to use dispersants,retardants, accelerators, extenders, foamers, defoamers, weightingagents, density-reducing additives and tensile strength-increasingfibers or silicate derivatives as auxiliary additives.

The invention further provides a method for cementing deep wells using acement slurry containing a mixture of the inventive polymer and starchin a concentration of 0.01% to 5% bwoc (by weight of cement), preferably0.05% to 0.9% bwoc. Further components of the cement slurry are watersof different salinity and cement. In addition, it is possible to usedispersants, retardants, accelerators, extenders, foamers, defoamers,weighting agents, density-reducing additives and tensilestrength-increasing fibers or silicate derivatives as auxiliaryadditives.

The term “starch” is understood to mean an organic compound. Starch is apolysaccharide having the formula (C₆H₁₀O₅)_(n) which consists ofα-D-glucose units joined to one another via glycosidic bonds. Starch mayconsist of:

-   -   0% to 100% by weight of amylose, linear chains having a helical        (screw) structure, having only α-1,4-glycosidic linkages, and    -   0% to 100% by weight of amylopectin, highly branched structures,        having α-1,6-glycosidic and α-1,4-glycosidic linkages. However,        the amylopectin of starch, with about one α-1,6-glycosidic bond        after about 30 α-1,4-glycosidic linkages, is less highly        branched than that of glycogen (about 1 α-1,6-glycosidic bond        for every 10 α-1,4-glycosidic bonds).

In this part of the world, starch is usually obtained from potatoes orcereals, but also from numerous other plants, such as rice (broken ricefrom rice dehusking factories) and corn. Another starch-providing plantof international significance is manioc (tapioca). In industrial starchproduction, various technologies are used according to the raw material.

In a particular embodiment of the invention, the starch can be thermallymodified in an upstream step. The starch can physically bind severaltimes its own weight of water, swell up and gelatinize under the actionof heat. When heated with water, the starch swells at 47-57° C., thelayers break up and gelatinized starch forms at 55-87° C. (potato starchat 62.5° C., wheat starch at 67.5° C.), having different stiffeningcapacity according to the starch type. The use of natural,non-pregelatinized starch has the advantage that, during the pumping ofa cement slurry, this cement slurry is rheologically stabilized when thetemperature increases.

In a further particular embodiment of the invention, the starch may bein a chemically modified form. Modified starch, according to whichproperties are to be modified, is produced by various chemicalconversion processes. In the case of some modified starches, severalconversion processes are conducted in succession (e.g. acetylatedoxidized starch). Modified starches modified by chemical conversionprocesses are understood to mean:

-   -   acid-treated starch by reaction with acids (for example with        hydrochloric acid, phosphoric acid or sulfuric acid)    -   alkali-modified starch by reaction with alkalis (for example        with sodium hydroxide solution or potassium hydroxide solution)    -   bleached starch by treatment with peroxyacetic acid, hydrogen        peroxide, sodium hypochlorite, sodium chlorite, sulfur dioxide,        sulfites, potassium permanganate or ammonium persulfate    -   enzymatically modified starch by treatment with amylases    -   oxidized starch by oxidation (for example with sodium        hypochlorite)    -   monostarch phosphate by esterification with phosphorous ester        groups (for example phosphoric acid, sodium phosphate or        potassium phosphate, phosphonic acid or pentasodium        triphosphate)    -   distarch phosphate by esterification with sodium        trimetaphosphate or phosphorus oxychloride    -   phosphated distarch phosphate by combination of the processes        for preparing monostarch phosphate and distarch phosphate    -   acetylated starch by esterification (for example with        anhydrides)    -   hydroxypropyl starch by reaction with propylene oxide    -   sodium octenylsuccinate starch by reaction of starch with        octenylsuccinic anhydride.

The most commonly desired improvements are in resistance to heat, coldand/or pH changes (acids).

The invention therefore further provides mixtures comprising theabovementioned starch and/or modifications thereof and the polymers ofthe invention. These mixtures preferably contain:

-   25% to 75% by weight of starch and-   25% to 75% by weight of the polymers of the invention.

In a particular embodiment, the mixtures contain

-   25% to 75% by weight of a chemically modified starch and-   25% to 75% by weight of the polymers of the invention.

In a further particular embodiment, the mixtures contain

-   25% to 75% by weight of a thermally modified starch and-   25% to 75% by weight of the polymers of the invention.

An inventive mixture of starch and the polymers of the invention can beobtained during the polymerization process after the actualpolymerization and before the drying operation or isolation operation.

A further inventive mixture of starch and the polymers of the inventioncan be obtained by the mixing of the pulverulent starch with thepulverulent polymer of the invention.

EXAMPLES

In polymerization processes A to B described below, typical preparationprocesses for the polymers of the invention are described.

In the examples, there was variation of the polar solvent used, with theaid of which the polymers of the invention can be prepared. With the aidof polymerization methods A1 to A5 and B1 to B3, further polymers of theinvention were prepared by the variation of the monomers. These polymersand polymerization processes used for the synthesis thereof aresummarized in table 1a) to table 1i).

-   Polymerization process A1: Polymerization in    2-methylpropan-2-ol/water (3.5%) as polar solvent

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 772g of anhydrous 2-methyl-propan-2-ol are admixed with 28 g of distilledwater. The reaction vessel is within a thermostatted heating bath.

This reaction vessel is blanketed with nitrogen gas and, in a gentlenitrogen countercurrent, 113.2 g of acryloyldimethyltaurate areintroduced. The acryloyldimethyltaurate does not dissolve completely inthe 2-methylpropan-2-ol/water mixture and is partly in the form of adispersion of solids. The pH of this mixture is below pH 1. Through thegas inlet tube, gaseous ammonia is introduced above the liquid phaseuntil the pH of the dispersion is between 7 and 8. On attainment of thedesired pH range, the mixture is stirred for another 1 hour and the pHis recorded continuously. The reaction vessel is blanketed with nitrogenand 79.2 g of acrylamide and 7.6 g of acrylic acid are introduced. Afterthe acrylamide has been introduced, the pH is checked again andoptionally corrected into the pH range of 7 to 8. A constant nitrogenstream is passed through the solution for at least 1 hour. After thisinertization time, the residual oxygen is checked by means of an oxygenelectrode. Should the measured value of residual oxygen in the liquidphase exceed the value of 1 ppm, another inertization is necessary untilthis value is attained. Thereafter, in a gentle nitrogen stream, 2 g of2,2′-azobis(2,4-dimethylvaleronitrile) are added and the reaction tankis heated to 40° C. Shortly after attainment of an internal temperatureof 40° C., the introduction of nitrogen gas is ended and commencement ofthe polymerization reaction is observed, which can be identified by atemperature increase of 10-35° C. About 5-15 minutes after onset of thepolymerization reaction, the temperature maximum has been exceeded andthe temperature in the reaction vessel is increased by means of theheating bath up to the boiling point of the 2-methylpropan-2-ol/watermixture. Under gentle reflux, the now viscous mixture is stirred for afurther two hours. The reaction product, present in the form of aviscous suspension of polymer in the 2-methylpropan-2-ol/water mixture,is removed by filtration and subsequent drying in a vacuum dryingcabinet.

-   Yield: 215.4 g of polymer 1-   Dry content (IR drier, 15 minutes at 120° C.): 94%-   K value (0.5% solution in distilled water): 212-   pH (0.5% solution in distilled water): 4.76-   Polymerization process A2: Polymerization in    2-methylpropan-2-ol/water (2%)

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 784g of anhydrous 2-methylpropan-2-ol are admixed with 16 g of distilledwater. The reaction vessel is within a thermostatted heating bath. Thefurther steps of polymerization process A2 are conducted analogously topolymerization process A1.

With the aid of polymerization method A2, further polymers of theinvention were prepared by varying the monomers. These polymers aresummarized in table 1.

-   Polymerization process A3: Polymerization in    2-methylpropan-2-ol/water (4.5%)

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 764g of anhydrous 2-methylpropan-2-ol are admixed with 36 g of distilledwater. The reaction vessel is within a thermostatted heating bath. Thefurther steps of polymerization process A3 are conducted analogously topolymerization process A1.

-   Polymerization process A4: Polymerization in    2-methylpropan-2-ol/water (1.5%)

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 788g of anhydrous 2-methylpropan-2-ol are admixed with 12 g of distilledwater. The reaction vessel is within a thermostatted heating bath. Thefurther steps of polymerization process A4 are conducted analogously topolymerization process A1.

-   Polymerization process A5: Polymerization in    2-methylpropan-2-ol/water (7.5%)

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 748g of anhydrous 2-methylpropan-2-ol are admixed with 52 g of distilledwater. The reaction vessel is within a thermostatted heating bath. Thefurther steps of polymerization process A5 are conducted analogously topolymerization process A1.

-   Polymerization process B1: Polymerization in 2-methylpropan-2-ol,    dimethyl ketone and water as polar solvent (50:50, 3.4% water)

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 384g of anhydrous 2-methylpropan-2-ol and 384 g of dimethyl ketone areadmixed with 26.4 g of distilled water. The reaction vessel is within athermostatted heating bath.

This reaction vessel is blanketed with nitrogen gas and, in a gentlenitrogen countercurrent, 130 g of acryloyldimethyltaurate areintroduced. The acryloyldimethyltaurate does not dissolve completely inthe 2-methylpropan-2-ol/dimethyl ketone/water mixture and is partly inthe form of a dispersion of solids. The pH of this mixture is belowpH 1. Through the gas inlet tube, gaseous ammonia is introduced abovethe liquid phase until the pH of the dispersion is between 7 and 8. Onattainment of the desired pH range, the mixture is stirred for another 1hour and the pH is recorded continuously. The reaction vessel isblanketed with nitrogen and 60 g of acrylamide and 10 g of acrylic acidare introduced. After the acrylamide has been introduced, the pH ischecked again and optionally corrected into the pH range of 7 to 8. Aconstant nitrogen stream is passed through the solution for at least 1hour. After this inertization time, the residual oxygen is checked bymeans of an oxygen electrode. Should the measured value of residualoxygen in the liquid phase exceed the value of 1 ppm, anotherinertization is necessary until this value is attained. Thereafter, in agentle nitrogen stream, 2.05 g of 2,2′-azobis(2,4-dimethylvaleronitrile)are added and the reaction tank is heated to 40° C. Shortly afterattainment of an internal temperature of 40° C., the introduction ofnitrogen gas is ended and commencement of the polymerization reaction isobserved, which can be identified by a temperature increase of 10 to 35°C. About 5-15 minutes after onset of the polymerization reaction, thetemperature maximum has been exceeded and the temperature in thereaction vessel is increased by means of the heating bath up to theboiling point of the 2-methylpropan-2-ol/water mixture. Under gentlereflux, the now viscous mixture is stirred for a further two hours. Thereaction product, present in the form of a viscous suspension of polymerin the 2-methylpropan-2-ol/water mixture, is removed by filtration andsubsequent drying in a vacuum drying cabinet.

-   Polymerization process B2: Polymerization in 2-methylpropan-2-ol,    dimethyl ketone and water as polar solvent (75:25, 3.0% water)

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 582g of anhydrous 2-methylpropan-2-ol and 194 g of dimethyl ketone areadmixed with 24 g of distilled water. The reaction vessel is within athermostatted heating bath. The further steps of polymerization processB2 are conducted analogously to polymerization process B1.

-   Polymerization process B3: Polymerization in 2-methylpropan-2-ol,    dimethyl ketone and water as polar solvent (25:75, 5.0% water)

In a 2 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 194g of anhydrous 2-methylpropan-2-ol and 582 g of dimethyl ketone areadmixed with 24 g of distilled water. The reaction vessel is within athermostatted heating bath. The further steps of the polymerizationprocess are conducted analogously to polymerization process B1.

TABLE 1a) Examples of polymers of the invention prepared bypolymerization processes A1 to A5 and B1 to B3 ACDMT Acrylic acidAcrylamide V-65 ® Polymer Process g mol % g mol % g mol % g % by wt. kvalue 2 A1 110.0 29.6 10.00 7.7 80.0 62.7 2.00 1.0 215 4 A1 113.2 30.97.60 6.0 79.2 63.1 2.00 1.0 210 6 A1 98.0 32.0 6.40 6.0 65.0 61.9 1.901.1 208 7 A1 119.0 32.5 5.70 4.5 79.2 63.0 2.25 1.1 200 8 A1 104.5 28.59.60 7.5 80.4 64.0 2.25 1.2 204 12 A2 110.0 29.6 10.00 7.7 80.0 62.72.25 1.1 192 13 A2 113.2 30.9 7.60 6.0 79.2 63.1 2.20 1.1 188 14 A2119.0 32.5 5.70 4.5 79.2 63.0 2.20 1.1 179 16 A3 113.2 30.9 7.60 6.079.2 63.1 2.20 1.1 216 18 A3 110.0 29.6 10.00 7.7 80.0 62.7 2.25 1.1 22220 A4 90.0 25.1 6.20 5.0 86.0 69.9 2.18 1.2 176

TABLE 1b) Examples of polymers of the invention prepared bypolymerization processes A1 to A5 and B1 to B3 ACDMT Acrylic acidAcrylamide V-65 ® Polymer Process g mol % g mol % g mol % g % by wt. kvalue 25 B1 109.5 29.5 9.70 7.5 80.3 63.0 2.30 1.2 203 26 B1 100.0 28.56.10 5.0 80.0 66.5 2.10 1.1 211 28 B1 113.2 31.0 7.60 6.0 79.0 63.0 2.201.1 213 31 B2 99.5 34.5 8.00 8.0 56.8 57.5 1.80 1.1 219 33 B2 110.0 34.33.40 3.0 69.0 62.7 2.00 1.1 227 34 B2 119.0 32.5 5.70 4.5 79.2 63.0 2.251.1 228 35 B2 110.0 29.6 10.00 7.7 80.0 62.7 2.25 1.1 221 36 B2 113.230.9 7.60 6.0 79.2 63.1 2.25 1.1 227 38 B3 113.2 30.9 7.60 6.0 79.2 63.12.20 1.1 216 40 B3 110.0 29.6 10.00 7.7 80.0 62.7 2.25 1.1 218

TABLE 1c) Polymers of the invention by polymerization process A1 ACDMTMethacrylic Acrylamide V-65 ® k Polymer Process mol % acid mol % mol % gvalue 42 A1 30 7 63 2.00 208 44 A1 31 6 63 2.00 219

TABLE 1d) Polymers of the invention by polymerization process A1Dimethyl- ACDMT Methacrylic acrylamide V-65 ® k Polymer Process mol %acid mol % mol % g value 47 A1 30 7 63 2.0 201 49 A1 31 6 63 2.0 198

TABLE 1e) Polymers of the invention by polymerization process A1 ACDMTCEA Acrylamide V-65 ® Polymer Process mol % mol % mol % g k value 52 A130 7 63 2.0 217 54 A1 31 6 63 2.0 209

TABLE 1f) Polymers of the invention by polymerization process A1Dimethyl- ACDMT CEA acrylamide V-65 ® Polymer Process mol % mol % mol %g k value 57 A1 30 7 63 2.0 218 59 A1 31 6 63 2.0 207

TABLE 1g) Polymers of the invention by polymerization process A1 CEAACDMT oligomer Acrylamide V-65 ® Polymer Process mol % mol % mol % g kvalue 62 A1 30 7 63 2.0 214 64 A1 31 6 63 2.0 205

TABLE 1h) Polymers of the invention by polymerization process A1 CEADimethyl- ACDMT oligomer acrylamide V-65 ® Polymer Process mol % mol %mol % g k value 67 A1 30 7 63 2.0 208 69 A1 31 6 63 2.0 218

-   ACDMT=acryloyldimethyltaurate-   AA=acrylic acid-   AM=acrylamide-   CEA=carboxyethyl acrylate-   CEA oligomer=carboxyethyl acrylate oligomer mixture with n=0 to 5-   V-65=2,2′-azobis(2,4-dimethylvaleronitrile)/V-65 is a registered    trademark of Wako Pure Chemicals Industries, Ltd-   Comparative example 1 (not in accordance with the invention,    prepared according to EP-1045869, copolymer prepared in    precipitation polymerization, 44.5 mol % of acryloyldimethyltaurate    and 55.5 mol % of acrylamide)

In a 3 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube,1700 g of anhydrous 2-methylpropan-2-ol are admixed with 50 mL ofdistilled water. The reaction vessel is within a thermostatted heatingbath.

This reaction vessel is blanketed with nitrogen gas and, in a gentlenitrogen countercurrent, 245 g of acryloyldimethyltaurate areintroduced. The acryloyldimethyltaurate does not dissolve completely inthe 2-methylpropan-2-ol/water mixture and is partly in the form of adispersion of solids. The pH of this mixture is below pH 1. Through thegas inlet tube, gaseous ammonia is introduced above the liquid phaseuntil the pH of the dispersion is between 7 and 8. On attainment of thedesired pH range, the mixture is stirred for another 1 hour and the pHis recorded continuously. The reaction vessel is blanketed with nitrogenand 105 g of acrylamide are introduced. After the acrylamide has beenintroduced, the pH is checked again and optionally corrected into the pHrange of 7 to 8. A constant nitrogen stream is passed through thesolution for at least 1 hour. After this inertization time, the residualoxygen is checked by means of an oxygen electrode. Should the measuredvalue of residual oxygen in the liquid phase exceed the value of 1 ppm,another inertization is necessary until this value is attained.Thereafter, in a gentle nitrogen stream, 2 g of AIBN are added and thereaction tank is heated to 60° C. Shortly after attainment of aninternal temperature of 60° C., the introduction of nitrogen gas isended and commencement of the polymerization reaction is observed, whichcan be identified by a temperature increase of 10-15° C. About 5-15minutes after onset of the polymerization reaction, the temperaturemaximum has been exceeded and the temperature in the reaction vessel isincreased by means of the heating bath up to the boiling point of the2-methylpropan-2-ol/water mixture. Under gentle reflux, the now viscousmixture is stirred for a further two hours. The reaction product,present in the form of a viscous suspension of polymer in the2-methylpropan-2-ol/water mixture, is removed by filtration andsubsequent drying in a vacuum drying cabinet.

-   Yield: 365 g-   Dry content (IR drier, 15 minutes at 120° C.): 96%-   K value (0.5% solution in distilled water): 212-   Comparative example 2 (not in accordance with the invention,    prepared according to EP-0244981, copolymer prepared in an aqueous    gel polymerization, 18.6 mol % of acryloyldimethyltaurate, 10 mol %    of acrylic acid and 71.3 mol % of acrylamide)

In EP-0244981, reference is made in the examples to a gel polymerizationin a conventional manner. No detailed preparation process for thepolymers in EP-0244981 is described.

For comparative example 2, a 1 liter Quickfit flask with anchor stirrer,reflux condenser with offgas scrubber, combined thermometer/pH meter anda gas inlet tube was initially charged with 390 g of distilled water, 40g of acryloyldimethyltaurate, 7.5 g of acrylic acid and 52.5 g ofacrylamide. Nitrogen gas is passed through the reaction solution for 1hour. Thereafter, 2 g of ammonium peroxodisulfate dissolved in 10 g ofdistilled water are added as initiator. This mixture is heated to 40° C.until a polymerization reaction occurs after 10-15 minutes. Afterpassing through the temperature maximum, the internal temperature isadjusted to 60° C. by means of the thermostat. A clear gel of highviscosity forms. The gel is comminuted mechanically and dried in avacuum drying cabinet.

-   Comparative example 3 (not in accordance with the invention,    prepared according to EP-0244981, copolymer prepared in an aqueous    gel polymerization, 34 mol % of acryloyldimethyltaurate, 11.4 mol %    of acrylic acid and 54.6 mol % of acrylamide).

For comparative example 3, a 1 liter Quickfit flask with anchor stirrer,reflux condenser with offgas scrubber, combined thermometer/pH meter anda gas inlet tube was initially charged with 390 g of distilled water, 60g of acryloyldimethyltaurate, 7 g of acrylic acid and 11.4 g ofacrylamide. Nitrogen gas is passed through the reaction solution for 1hour. Thereafter, 2 g of ammonium peroxodisulfate dissolved in 10 g ofdistilled water are added as initiator. This mixture is heated to 40° C.until a polymerization reaction occurs after 10-15 minutes. Afterpassing through the temperature maximum, the internal temperature isadjusted to 60° C. by means of the thermostat. A clear gel of highviscosity forms. The gel is comminuted mechanically and dried in avacuum drying cabinet.

-   Comparative example 4 (not in accordance with the invention,    prepared according to EP-0244981, copolymer prepared in an aqueous    gel polymerization, 10.3 mol % of acryloyldimethyltaurate, 5.9 mol %    of acrylic acid and 84.9 mol % of acrylamide). Comparative example 4    was prepared analogously to comparative example 2.

For comparative example 4, a 1 liter Quickfit flask with anchor stirrer,reflux condenser with offgas scrubber, combined thermometer/pH meter anda gas inlet tube was initially charged with 390 g of distilled water, 25g of acryloyldimethyltaurate, 5 g of acrylic acid and 70 g ofacrylamide. Nitrogen gas is passed through the reaction solution for 1hour. Thereafter, 2 g of ammonium peroxodisulfate dissolved in 10 g ofdistilled water are added as initiator. This mixture is heated to 40° C.until a polymerization reaction occurs after 10-15 minutes. Afterpassing through the temperature maximum, the internal temperature isadjusted to 60° C. by means of the thermostat. A clear gel of highviscosity forms. The gel is comminuted mechanically and dried in avacuum drying cabinet.

-   Comparative example 5 (not in accordance with the invention,    prepared according to U.S. Pat. No. 4,015,991, copolymer prepared in    an aqueous gel polymerization, 10.3 mol % of    acryloyldimethyltaurate, 5.9 mol % of acrylic acid and 84.9 mol % of    acrylamide).

A 3 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube isinitially charged with 328 g of distilled water and 116.4 g ofacryloyldimethyltaurate. The acryloyldimethyltaurate is neutralized byadding 45 g of a 50% sodium hydroxide (NaOH) solution. After theneutralization reaction, a clear solution having a pH between 7 and 8 isobtained. 14.7 g of acrylamide are dissolved gradually in the solutionthus neutralized. Nitrogen gas is passed through the reaction solutionfor 1 hour. Thereafter, 0.69 g of tert-butyl peroxypivalate and 1.0 mLof an iron ammonium sulfate solution as redox initiator pair are added.The iron ammonium sulfate solution is prepared by dissolving 0.098 g ofFe(NH₄)₂(SO₄)₂ in 500 g of water. This mixture is additionally stirredat room temperature until, after 1-2 hours, a polymerization reactionoccurs. The exothermic polymerization reaction, in the case of adiabaticpolymerization, increases the temperature to 50-60° C. After passingthrough the temperature maximum, the internal temperature is adjusted to60° C. by means of the thermostat. A clear gel of high viscosity isformed. The gel is comminuted mechanically and dried on a roller drier.

-   Yield: 152 g of comparative polymer 5

According to the testing in U.S. Pat. No. 4,015,991, this base polymershould have only poor water-loss reducing action. In contrast, at lowtemperatures, 28° C., the partly hydrolyzed products should have goodperformance properties. These products were prepared and tested as incomparative example 6.

-   Comparative example 6 (not in accordance with the invention,    prepared according to U.S. Pat. No. 4,015,991) Controlled hydrolysis    of comparative polymer 5

45.3 g of comparative polymer 5 are dissolved in 1500 mL of distilledwater while stirring. On completion of dissolution of the polymer, 1.68g of potassium hydroxide which have been dissolved in 20 mL of waterbeforehand are added thereto. This mixture is heated to 60° C. thenstirred at this temperature for one hour. The reaction product is driedagain with the aid of a roller drier. In this way, 50% hydrolysis isachieved.

Examples: Synergistic Mixtures Comprising Starch and the Polymers of theInvention

For the synergistic mixtures comprising starch and the polymers of theinvention, the following starch types were used:

-   Starch A: corn starch-   Starch B: manioc starch-   Starch C: “cook up modified starch”-   Starch D: hydroxypropyl starch

Table 2 describes the mixtures made with the starches A to E and thepolymers of the invention.

TABLE 2 Synergistic mixtures comprising starch and the polymers of theinvention Mixture Starch % by wt. Polymer % by wt. 1 A 30  (4) 70 2 A 50 (4) 50 3 A 70  (4) 30 4 B 50  (4) 50 5 D 60  (4) 40 6 A 25 (42) 75 7 A40 (42) 60 8 A 65 (42) 35 9 B 25 (42) 75 10 B 40 (42) 60 11 B 65 (42) 3512 B 50 (52) 50 13 C 50 (52) 50 14 D 50 (52) 50 15 D 40 (52) 60 16 D 60(52) 40 17 A 75  (7) 25 18 D 50  (7) 50 25 A 45 (26) 55 26 E 65 (26) 3527 A 50 (49) 50 28 A 45 (49) 50 29 A 65 (49) 35 30 B 50 (49) 50 31 C 45(49) 50 32 D 65 (49) 35 33 A 50 (54) 50 34 B 50 (54) 50 35 D 50 (54) 50

Examples: Test Results

The testing is effected according to API spec. 10. In an atmosphericconsistometer, the cement slurry is stirred/conditioned at analysistemperature and then, at the same temperature, the rheology with theFANN model 35SA viscometer is measured (at high temperature,conditioning is effected at 93° C. and the viscosity is measured). Attemperatures of >93° C., the water loss is measured with a stirringfluid loss apparatus (SFLA).

Table 3 shows the water loss-reducing properties of selectedabovementioned examples according to API spec. 10 at 35° C. (95° F.) ina static filtration test in the Baroid HTHP filter press. It becomesclear that the water loss reduction with the polymers of the inventioncan be improved considerably at low temperatures compared to thecomparative examples. Of course, at these low temperatures, the polymersbased on acryloyldimethyltaurate and acrylamide as claimed inEP-1045869, reworked in comparative example 1, also reduce water loss.However, it becomes clear from table 1 that the water loss ofcomparative example 1 is nearly twice as high compared to the inventivepolymer 4. The gel polymers described in EP-0244981, reworked incomparative examples 2 to 3, show a high water loss even at very lowtemperatures and are unsuitable for use. For this reason, comparativeexamples 2 to 3 were not considered in the measurements which follow.The two comparative examples from U.S. Pat. No. 4,015,991, based onpartly hydrolyzed poly(acrylamide-co-acryloyldimethyltaurate), reworkedin comparative examples 5 and 6, in direct comparison with polymer 4,likewise have nearly twice to three times the water loss. These polymerstoo appear unsuitable for use.

Formulation of the Cement Slurries:

-   100 g of Dyckerhoff Class G-   44 g of distilled water-   0.3-0.5 g of polymer

TABLE 3 (Use test at 95° F. (35° C.)) Rheology after mixing at 75° F.(24° C.), scale divisions API at X revolutions per minute fluid PolymerConcentration Revolutions per minute (rpm) loss No. % by wt. 600 300 200100 6 3 mL 2 (P) 0.5 95 74 46 33 6 3 60 4 (P) 0.3 108 77 44 30 5 3.5 588 (P) 0.3 103 73 39 28 4 3 72 13 (P) 0.3 90 72 38 25 4 3 75 16 (P) 0.3110 87 53 36 5 3.5 95 18 (P) 0.3 95 73 45 26 4 3 85 20 (P) 0.5 88 67 4728 4.5 3.5 66 28 (P) 0.3 87 69 39 26 5 3 65 33 (P) 0.3 98 82 59 30 6 3.570 36 (P) 0.3 86 63 42 30 5 3.5 68 1 (C) 0.3 94 68 39 26 5 3 115  2 (C)0.3 290 167 118 67 7.5 4.5 470* 3 (C) 0.3 120 105 77 56 11 5 660* 4 (C)0.3 275 151 116 59 9 6.5 270* 5 (C) 0.3 168 88 68 38 11 11.5 143* 6 (C)0.3 225 117 84 43 7 4.5 178* (P) = inventive polymer (ex.: 30 (P) =inventive polymer 30 from Table 1a)) (C) = non-inventive comparativeexample (ex. 5 (C) = comparative example 5) *values are calculated,since all the water had been expressed before the test had ended.

Table 4 shows the water loss-reducing properties of selectedabovementioned examples according to API spec. 10 at 121.1° C. (250° F.)in a stirred filtration test in the Fann HTHP filter press (stirringfluid loss apparatus, SFLA). In order to better show the improvedproperties of the polymers of the invention compared to the state of theart of the polymers claimed in EP1045869 (comparative example 1), theconcentration of the polymers used was varied between 0.25% and 0.5% byweight. It becomes clear that, with the polymers of the invention atlower concentrations (0.25% by weight), water loss is reduced by 40%compared to by weight (80 mL) the claimed polymers in EP 1045869 (130mL).

Formulation of the Cement Slurries:

-   100 g of Dyckerhoff Class G Cement-   35 g of silica flour-   54.8 g of distilled water

Polymer in the Concentration Specified in Table 3 or 4

-   0.3 g of dispersant (polynaphthalenesulfonate, PNS)-   0.5 g of retarder (lignosulfonate)

TABLE 4 (Use test at 250° F. (121.1° C.)) Rheology after mixing at 75°F. (24° C.), scale divisions API at X revolutions per minute fluidPolymer Concentration Revolutions per minute (rpm) loss No. % by wt. 600300 200 100 6 3 mL 1 (C) 0.25 167 91 63 34 5 3.5 130 1 (C) 0.5 >300 168117 64 7.5 5.0 52 4 (P) 0.25 170 91 63 34 5 3.5 80 4 (P) 0.5 >300 174119 65 7 4.5 42 2 (P) 0.5 295 164 113 62 7 4.5 44 4 (P) 0.5 >300 174 11965 7 4.5 42 13 (P) 0.5 >300 179 126 70 8 5 40 7 (P) 0.5 >300 174 123 687 4 43 31 (P) 0.5 >300 167 121 58 7.5 3.5 54 26 (P) 0.5 295 173 119 64 74.5 60 (P) = inventive polymer (ex.: 30 (P) = inventive polymer 30 fromTable 1a))

Table 5 shows the water loss-reducing properties of selectedabovementioned mixture of starch and the polymers of the inventionaccording to API spec. 10 under various temperature conditions (250° F.,300° F. and 350° F.) in a stirred filtration test in the Fann HTHPfilter press (stirring fluid loss apparatus, SFLA).

Formulation of the Cement Slurries:

-   100 g of Dyckerhoff Class G Cement-   35 g of silica flour-   54.8 g of distilled water

Polymer in the Concentration Specified in Table 3 or 4

-   0.3 g of dispersant (polynaphthalenesulfonate, PNS)-   0.5-1.5 g of retarder (lignosulfonate)

TABLE 5 (Use tests of the mixtures at different temperatures) Rheologyafter mixing at 75° F. (24° C.), API scale divisions at X revolutionsper minute fluid Temp. Revolutions per minute (rpm) loss Mixture ° F.600 300 200 100 6 3 mL 2 250 188 104 71 38 5 4 102 4 250 173 93 63 34 65 106 3 250 208 115 79 43 6 4.5 60 2 300 191 108 74 42 8.5 9 52 4 300203 113 78 43 7.5 6.5 46 3 300 242 133 94 52 9 7.5 43 3 300 228 127 8849 9 7.5 41 2 350 206 119 91 50 15 14 96 3 350 217 123 89 54 13.5 12.592 (P) = inventive polymer (ex.: 30 (P) = inventive polymer 30 fromtable 1a))

It was therefore an object of the present invention to provide polymerswhich can help to achieve improved control of liquid loss in the cementslurries for cementing wellbores at temperatures between 80° F. and 300°F. WO-99/26991 describes copolymers of AMPS and acrylamide. Table 4 onpage 23 discloses that there is a distinct decline in the waterloss-reducing properties of the polymers described within a temperaturerange between 100° F. and 200° F., and doubling of the water loss insome cases in the use test. The addition of acrylic acid in the polymersof the invention distinctly improves the water loss compared to thepolymers of WO-99/26991. The comparative examples which follow areintended to demonstrate this.

Comparative Examples According to WO-99/26991

-   Comparative polymer 7: (not in accordance with the invention,    prepared according to WO-99/26991—copolymer of    acryloyldimethyltaurate 70% by weight and acrylamide 30% by weight)

In a 3 liter Quickfit flask with anchor stirrer, reflux condenser withoffgas scrubber, combined thermometer/pH meter and a gas inlet tube, 50ml of distilled water are added to 1700 g of anhydrous2-methylpropan-2-ol. The reaction vessel is in a thermostated heatingbath.

This reaction vessel is blanketed with nitrogen gas and, in a gentlenitrogen countercurrent, 245 g of acryloyldimethyltaurate areintroduced. The acryloyldimethyltaurate does not dissolve completely inthe 2-methylpropan-2-ol/water mixture and is partly in the form of adispersion of solids. The pH of this mixture is below pH 1. Gaseousammonia is introduced through the gas inlet tube above the liquid phaseuntil the pH of the dispersion is between 7 and 8. On attainment of thedesired pH, the mixture is stirred for a further 1 hour and the pH isrecorded continuously. The reaction vessel is blanketed with nitrogen,and 105 g of acrylamide are introduced. After the acrylamide has beenintroduced, the pH is checked again and, if necessary, corrected to therange of pH 7 to 8. A constant nitrogen stream is passed through thesolution for at least 1 hour. After this inertization period, theresidual oxygen level is checked by means of an oxygen electrode. Shouldthe residual oxygen value in the liquid phase exceed the value of 1 ppm,another inertization is necessary until this value is attained.Thereafter, in a gentle nitrogen stream, 1.5 g of AIBN are added and thereaction vessel is heated to 60° C. Shortly after the attainment of aninternal temperature of 60° C., the introduction of nitrogen gas isended and commencement of the polymerization reaction is observed, whichcan be identified by an increase in temperature of 10 to 15° C. About5-15 minutes after onset of the polymerization reaction, the temperaturemaximum has been exceeded and the temperature in the reaction vessel isincreased by the heating bath up to the boiling point of the2-methylpropan-2-ol/water mixture. Under gentle reflux, the now viscousmass is stirred for a further two hours. The reaction product, in theform of a viscous suspension of polymer in the 2-methylpropan-2-ol/watermixture, is separated off by filtration and subsequent drying in avacuum drying cabinet.

-   Yield: 362 g-   Dry content (IR drier, 15 minutes at 120° C.): 97.5%-   K value (0.5% solution in distilled water): 208-   Comparative polymer 8 (not in accordance with the invention,    prepared according to WO-99/26991—copolymer of    acryloyldimethyltaurate 60% by weight and acrylamide 40% by weight)

The comparative example is prepared analogously to comparative polymer7. Rather than the amounts specified in comparative polymer 7, 210 g ofacryloyldimethyltaurate and 140 g of acrylamide are used.

-   Yield: 371 g-   Dry content (IR drier, 15 minutes at 120° C.): 95.5%-   K value (0.5% solution in distilled water): 219-   Comparative polymer 9 (not in accordance with the invention,    prepared according to WO-99/26991—copolymer of    acryloyldimethyltaurate 60% by weight and acrylamide 40% by weight)

The comparative example is prepared analogously to comparative polymer7. Rather than the amounts specified in comparative polymer 7, 280 g ofacryloyldimethyltaurate and 70 g of acrylamide are used.

-   Yield: 363 g-   Dry content (IR drier, 15 minutes at 120° C.): 96%-   K value (0.5% solution in distilled water): 201

Examples of Test Results

Testing is effected according to API spec. 10. In an atmosphericconsistometer, the cement slurry is stirred/conditioned at analysistemperature and then, at the same temperature, the rheology is measuredwith the FANN model 35SA viscometer (at high temperature, conditioningis effected at 93° C. and the viscosity is measured) and the water lossis measured at below 120° C. with a Baroid HTHP filter press and atabove 120° C. with the stirring fluid loss test apparatus. The settingtimes were determined with an Autoclave Engineers HTHP consistometer.Formulation of the cement slurries: ad 100% Dyckerhoff Class G Cement,43.7% distilled water, 0.3% polymer.

Table 6 shows the water loss-reducing properties of selectedabovementioned examples according to API spec. 10 at 35° C. (95° F.) ina static filtration test in the Baroid HTHP filter press. This makes itclear that it was possible with the polymers of the invention toconsiderably improve the reduction in water loss at low temperaturescompared to the comparative examples. Comparative example 7 differs fromcomparative example 1 merely by a somewhat smaller amount of initiator(1.5 g rather than 2.0); the measured K value for both polymersmentioned is identical to the value reported in WO-99/26991 and is 212.Both polymers were synthesized by the same method as a precipitationpolymer in tert-BuOH.

TABLE 6 API spec. 10 at 35° C. (95° F.) of the polymers of the inventioncompared to compared examples 1 amd 6 to 8 Rheology after mixing at 75°F. (24° C.), scale divisions API at X revolutions per minute fluidPolymer Concentration Revolutions per minute (rpm) loss No. % by wt. 600300 200 100 6 3 mL 2 (P) 0.5 95 74 46 33 6 3 60 4 (P) 0.3 108 77 44 30 53.5 58 8 (P) 0.3 103 73 39 28 4 3 72 13 (P) 0.3 90 72 38 25 4 3 75 20(P) 0.5 88 67 47 28 4.5 3.5 66 28 (P) 0.3 87 69 39 26 5 3 65 33 (P) 0.398 82 59 30 6 3.5 70 36 (P) 0.3 86 63 42 30 5 3.5 68 1 (C) 0.3 94 68 3926 5 3 115 7 (C) 0.3 96 72 41 26 4.5 3.5 109 8 (C) 0.3 113 81 47 29 6 3121 9 (C) 0.3 87 75 54 33 6 4 116 (P) = polymer (C) = comparativeexample

TABLE 7 API spec. 10 at 250° F. (121.1° C.) of the polymers of theinvention compared to comparative examples 1 and 6 to 8 Rheology aftermixing at 75° F. (24° C.), scale divisions API at X revolutions perminute fluid Polymer Concentration Revolutions per minute (rpm) loss No.% by wt. 600 300 200 100 6 3 mL 4 (P) 0.5 >300 174 119 65 7 4.5 42 2 (P)0.5 295 164 113 62 7 4.5 44 13 (P) 0.5 >300 179 126 70 8 5 40 7 (P)0.5 >300 174 123 68 7 4 43 31 (P) 0.5 >300 167 121 58 7.5 3.5 54 1 (C)0.5 >300 168 117 64 7.5 5.0 52 7 (C) 0.5 >300 175 113 67 7 5 54 8 (C)0.5 >300 187 129 72 8 6 60 9 (C) 0.5 >300 159 110 63 7 4.5 58 (P) =polymer (C) = comparative example

In table 6, it was possible to detect an improvement in the waterloss-reducing properties at 95° F. The water loss-reducing properties ofcomparative example 7 by direct comparison with comparative example 1show nearly identical values and demonstrate the reproducibility of theprocess used. Nevertheless, the water loss of comparative polymers 1 and6 to 9 at an average of 115 mL+/−5 mL is distinctly higher than that ofthe polymers of the invention. The difference in the water loss of thepolymers of the invention relative to the comparative polymers wasbetween 30 and 55 mL. This demonstrates clearly that the addition ofacrylic acid leads to a technical improvement at low temperatures.

Table 7 shows the water loss-reducing properties of selectedabovementioned examples according to API spec. 10 at 250° F. (121.1° C.)in a static filtration test in the Baroid HTHP filter press. In thistest too, it was possible to show that the polymers of the inventionhave a demonstrably lower water loss at temperatures between 80° F. and300° F. and under identical experimental conditions than the polymers ofWO-99/26991. Running the comparison between comparative example 7 ofWO-99/26991 and comparative example 1 (according to EP 1045869) again,the good reproducibility of the process used is shown in this test aswell. The use tests conducted according to API spec. 10 at 95° F. and250° F. demonstrate clearly that addition of acrylic acid to thepolymers of the invention leads to an improvement in the waterloss-reducing properties.

1. A water-soluble or water-swellable polymer having a k value of 100 to300, measured in 0.5% by weight solution in distilled water, containinga) 25-35 mol % of at least one recurrent structural unit of the formula(1)

in which R¹, R² are hydrogen, methyl or ethyl, A is linear or branchedC₁-C₁₂-alkylene, and Q⁺ is H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺, ½Ca⁺⁺, ½Mg⁺⁺, ½Zn⁺⁺,⅓Al⁺⁺⁺, organic ammonium ions of the formula [HNR⁵R⁶R⁷]⁺ where R⁵, R⁶and R⁷ may each independently be hydrogen, a linear or branched alkylgroup having 1 to 22 carbon atoms, a linear or branched, mono- orpolyunsaturated alkenyl group having 2 to 22 carbon atoms, aC₆-C₂₂-alkylamidopropyl group, a linear monohydroxyalkyl group having 2to 10 carbon atoms or a linear or branched dihydroxyalkyl group having 3to 10 carbon atoms, and where at least one of the R⁵, R⁶ and R⁷ radicalsis not hydrogen, or mixtures of these ions, b) 3 to 8 mol % of at leastone recurrent structural unit of the formula (2)

in which R¹ is hydrogen, methyl or ethyl, X⁺ is H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺,½Ca⁺⁺, ½Mg⁺⁺, ½Zn⁺⁺, ⅓Al⁺⁺⁺, organic ammonium ions of the formula[HNR⁵R⁶R⁷]⁺ where R⁵, R⁶ and R⁷ may each independently be hydrogen, alinear or branched alkyl group having 1 to 22 carbon atoms, a linear orbranched, mono- or polyunsaturated alkenyl group having 2 to 22 carbonatoms, a C₆-C₂₂-alkylamidopropyl group, a linear monohydroxyalkyl grouphaving 2 to 10 carbon atoms or a linear or branched dihydroxyalkyl grouphaving 3 to 10 carbon atoms, and where at least one of the R⁵, R⁶ and R⁷radicals is not hydrogen, or mixtures of these ions, B is a chemicalbond, or a linear or branched alkylene group having 1 to 6 carbon atoms,and n is an integer from 0 to 5, and c) 57 to 72 mol % of at least onerecurrent structural unit of the formula (3)

in which R¹ is hydrogen, methyl or ethyl, and R³ and R⁴ are eachindependently hydrogen, methyl, ethyl, n-propyl, isopropyl or butyl. 2.The polymer as claimed in claim 1, wherein the at least one structuralunit of the formula (1) is derived from monomers selected from the groupconsisting of acryloyldimethyltaurate,acryloyl-1,1-dimethyl-2-methyltaurate, acryloyltaurate, andacryloyl-N-methyltaurate.
 3. The polymer as claimed in claim 1, whereinthe at least one structural unit of the formula (2) is derived frommethacrylic acid, acrylic acid, carboxyethyl acrylate or higheroligomers of the formula (2) in which n is an integer of 2 or more. 4.The polymer as claimed in claim 1, wherein the at least one structuralunit of the formula (3) is derived from monomers selected from the groupconsisting of acrylamide, N-methylacrylamide, N-ethylacrylamide,N,N-diethylmethacrylamide, N,N-diethylacrylamide,N,N-dimethylmethacrylamide, N,N-dimethylacrylamide,N-isopropylacrylamide, N-tert-butylacrylamide and N-butylacrylamide. 5.The polymer as claimed in claim 1, wherein Q⁺ in formula (1) is selectedfrom the group consisting of NH₄ ⁺, Na⁺ and mixtures of these ions andX⁺ in formula (2) is selected from the group consisting of H⁺, NH₄ ⁺,Na⁺ and mixtures of these ions.
 6. The polymer as claimed in claim 1,containing 27.5 to 32.5 mol % of the structural units (1), 4.5 to 7.5 ofthe structural units (2) and 60 to 68 mol % of the structural units (3).7. The polymer as claimed in claim 1, in which the neutralization levelof the structural units of the formula (1) is 95 to 100 mol %.
 8. Thepolymer as claimed in claim 1, in which n=0 in at least 70% of all thestructural units of the formula (2).
 9. The polymer as claimed in claim1, in which n is 0 or
 1. 10. The polymer as claimed in claim 1, in whichformula (3) is derived from acrylamide.
 11. The polymer as claimed inclaim 1, which contains structural units of a) 27.5 to 32.5 mol % ofacryloyldimethyltaurate, b) 4.5 to 7.5 mol % of acrylic acid, and c) 60to 68 mol % of acrylamide.
 12. The polymer as claimed in claim 1, inwhich formula (1) represents acryloyldimethyltaurate, formula (2)represents acrylic acid and formula (3) represents acrylamide.
 13. Aprocess for preparing a polymer comprising the steps of polymerizing a)25-35 mol % of at least one recurrent structural unit of the formula (1)

in which R¹, R² are hydrogen, methyl or ethyl, A is linear or branchedC₁-C₁₂-alkylene, and Q⁺ is H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺, ½Ca⁺⁺, ½Mg⁺⁺, %Zn⁺⁺, ⅓Al⁺⁺⁺, organic ammonium ions of the formula [HNR⁵R⁶R⁷]⁺ where R⁵,R⁶ and R⁷ may each independently be hydrogen, a linear or branched alkylgroup having 1 to 22 carbon atoms, a linear or branched, mono- orpolyunsaturated alkenyl group having 2 to 22 carbon atoms, aC₆-C₂₂-alkylamidopropyl group, a linear monohydroxyalkyl group having 2to 10 carbon atoms or a linear or branched dihydroxyalkyl group having 3to 10 carbon atoms, and where at least one of the R⁵, R⁶ and R⁷ radicalsis not hydrogen, or mixtures of these ions, b) 3 to 8 mol % of at leastone recurrent structural unit of the formula (2)

in which R¹ is hydrogen, methyl or ethyl, X⁺ is H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺,½Ca⁺⁺, ½Mg⁺⁺, ½Zn⁺⁺, ⅓Al⁺⁺⁺, organic ammonium ions of the formula[HNR⁵R⁶R⁷]⁺ where R⁵, R⁶ and R⁷ may each independently be hydrogen, alinear or branched alkyl group having 1 to 22 carbon atoms, a linear orbranched, mono- or polyunsaturated alkenyl group having 2 to 22 carbonatoms, a C₆-C₂₂-alkylamidopropyl group, a linear monohydroxyalkyl grouphaving 2 to 10 carbon atoms or a linear or branched dihydroxyalkyl grouphaving 3 to 10 carbon atoms, and where at least one of the R⁵, R⁶ and R⁷radicals is not hydrogen, or mixtures of these ions, B is a chemicalbond, or a linear or branched alkylene group having 1 to 6 carbon atoms,and n is an integer from 0 to 5, and c) 57 to 72 mol % of at least onerecurrent structural unit of the formula (3)

in which R¹ is hydrogen, methyl or ethyl, and R³ and R⁴ are eachindependently hydrogen, methyl, ethyl, n-propyl, isopropyl or butyl viafree-radical precipitation polymerization in a polar solvent, and themonomers are optionally neutralized prior to the polymerization, or thepolymer is optionally neutralized after the polymerization, withammonia, ammonium carbonate or organic amines or an Li⁺-, Na⁺-, K⁺-,Ca⁺⁺-, Mg⁺⁺-, Zn⁺⁺- or Al⁺⁺⁺-containing base.
 14. The process as claimedin claim 13, wherein the polar solvent has a boiling point of 60 to 110°C.
 15. The process as claimed in claim 13, wherein the polar solvent isa solvent mixture of d) water and e) at least one further polar solvent.16. The process as claimed in claim 13, wherein the polar solventcomprises methanol, ethanol, 1-propanol, 2-propanol,2-methyl-2-propanol, 1-butanol, 2-butanol, dimethyl ketone, diethylketone, pentan-2-one, butanone, tetrahydropyran, tetrahydrofuran,2-methyltetrahydrofuran, 1,3-dioxane or 1,4-dioxane.
 17. The process asclaimed in claim 13, in which the polar solvent comprises2-methylpropan-2-ol.
 18. The process as claimed in claim 13, wherein thepolar solvent comprises 1% to 5% by weight of water.
 19. The process asclaimed in claim 13, wherein 27.5 to 32.5 mol % ofacryloyldimethyltaurate, and 4.5 to 7.5 mol % of acrylic acid, 60 to 68mol % of acrylamide is subjected to free-radical precipitationpolymerization a mixture of 1% to 8% by weight of water and 92% to 99%by weight of 2-methylpropan-2-ol, and the monomers prior to thepolymerization or the polymer after the polymerization are/is optionallyneutralized with ammonia, ammonium carbonate, sodium hydroxide, sodiumcarbonate.
 20. (canceled)
 21. (canceled)
 22. A method for cementing deepwells using a cement slurry, wherein the cement slurry comprises apolymer as claimed in claim
 1. 23. The method as claimed in claim 22, inwhich the polymer content is 0.05% to 5% by weight, based on the cementslurry.
 24. A polymer mixture containing 25% to 75% by weight of apolymer as claimed in claim 1, and 25% to 75% by weight of starch,chemically modified starch or thermally modified starch.